This invention relates generally to a method for manufacturing bicycle frames, in particular, to a method for molding bicycle frames by using composite materials and molds suitable for implementing the method.
The performance characteristics of composite bicycle frames which conventional bicycle frames cannot achieve are light weight, high specific strength (strength/specific gravity), high specific rigidity (rigidity/specific gravity), suitable for versatile design, good ability for absorbing shock, etc. Therefore the manufacturing of composite bicycle frames has become one of the aims in the development of bicycles in the world.
The composite bicycle frames can be divided into two kinds: the lug type and integrally formed type. Bicycle frames of the lug type are made from individual composite tubes which are joined together by adhering metallic joints therebetween. Although they have the advantages of easy mass production and low cost, they still have the disadvantages of limited configuration and low reliability of adhesive strength, that is, the stress transferred between joints and composite tubes is only achieved by adhesive therebetween so that the strength is relatively weak. Hence, a monocoque type frame has been developed to overcome the preceding drawbacks, that is, the whole frame is made from composite materials with metallic parts embedded in suitable positions, if necessary.
Conventionally, a bicycle frame, as shown in FIG. 1, comprises steering support means 10, seat support means 20, pedal support means 30, a pair of rear wheel dropouts 40, a top tube 50, a down tube 60, a seat tube 70, a pair of seat stays 80 and a pair of chain stays 90. The top tube 50, down tube 60, and seat tube 70 form an open "front triangle", and the seat tube 70, seat stays 80 and chain stays 90 form a pair of open "rear triangles", with the seat tube 70 being the common side of the front and rear triangles. It is to be noted that the bicycle frames can have many variations in configuration, for example, some omit the seat tube, and some do not have a definite top tube. Generally, the triangle formed by the steering support means 10, seat support means 20 and pedal support means 30 is still called a "front triangle", and the seat stays 80 and chain stays 90 can be transformed to form a pair of continuous "rear triangles". The bicycle frame described in the specification is directed to a combination of the front triangle and rear triangles.
In molding the composite bicycle frames, plies of prepreg (glass fibers, carbon fibers or KEVLAR.RTM. fibers) can be laid up depending upon the required shape in each of the mold pieces and be heated and cured to form the desired bicycle frame.
Regarding the methods for integrally forming the composite bicycle frames, they are disclosed in U.S. Pat. Nos. 4,850,607, 4,889,355 and 4,902,458 (all by the same inventor). These prior patents disclosed the method of making composite bicycle frames by using three mold pieces (referring to FIG. 2, the reference numerals 97, 98 and 99 identify the three mold pieces, respectively.) In molding the frame by the three mold pieces, pieces of resin impregnated fabric are laid up and air bladders are placed therein. The uncured frame is thereafter placed in the three mold pieces to be inflated, heated and cured. When inflating, the pressure generated by the inflated bladders may be insufficient to press some places which are near the steering support means, seat support means and pedal support means of the bicycle frame. Therefore foamable resins which expand on heating are used to provide sufficient pressure for forming these portions.
However, these frames in the prior patents have the following drawbacks:
(a) The method of molding the frame structure employs a female molding unit having three mold pieces. It is difficult to control the molding accuracy and the manufacture of the molding unit is expensive because of its large size. PA1 (b) It is difficult to transfer heat from the molding unit to the frame such that the molding process is slow and the frame has a longer molding cycle resulting in reduced production because the molding unit is large in size. PA1 (c) It is inconvenient to place the laid up impregnated materials in the molding cavities and to assemble the separate mold pieces together because of the large-sized mold pieces. The impregnated materials are easily pinched by the mold pieces. PA1 (d) The frame which is formed by using the foamable resins will increase its weight. PA1 (e) The bubbles in the foamable resins which remain in the bicycle frame will expand to a rather large extent due to the heating and cause stress concentrations between the frame. PA1 (f) It is difficult to control the foamable resins and they will flow out of the clearances between the impregnated materials and sleeves embedded in the frame.